In a typical laser-based printing/copying process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of the original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charges thereon in the irradiated areas. This records a latent image on the photoconductive member corresponding to the informational areas contained within the original document.
One approach to the fixing of toner images onto a support has been to pass the support bearing the toner images between a pair of opposed roller members, at least one of which is internally heated. During operation of a fixing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls and thereby heated under pressure. A large quantity of heat is applied to the toner and the copy sheet bearing the toner image. This heat evaporates much of the moisture contained in the sheet. The quantity of heat applied to the front and to the back sides of the sheet is often not equal.
One problem associated with moisture loss in paper is paper curl. Deformation of the paper generally occurs due to a change of physical properties in the z-direction of paper. This may occur through the following event: as sheets pass through an image fixing system, moisture is driven out and the sheet temperature is elevated. After image fixing, a sheet typically rests in a collection area exposed to its ambient surroundings, where its moisture content will reach equilibrium with the environment through absorption of moisture across the full face of at least one side of the paper sheet. However, if the copy sheet becomes part of a large compiled set, both sides of all of the papers in the compilation (except for the top sheet) will effectively be sealed off from ambient moisture. The only route available to the papers for moisture re-absorption is through the edges of the sheets, leaving the moisture content of the central portions of the sheets relatively unchanged. This uneven pattern of moisture re-absorption results in edge stresses that lead to paper curl along the edges of the paper.
Further, contact with moisture can cause curl prior to image fixing. Thus, in addition to being cosmetically unsightly, the curl creates a handling problem, in that pages with a wave pattern along their edges are more difficult to feed to secondary paper handling machines. It is especially important to papers which are used in automatic sheet-fed printing operations such as xerography. Excessive curl can cause the paper transport mechanism to jam, thereby creating operator frustration, lost time and service expense. Therefore, curl is an important mechanical property of non-woven webs such as paper which manufacturers seek to minimize, and there is a continuing need for a measuring device to predict the curl performance of webs which will be used in sheet-fed machines.
Moreover, in sheet-fed apparatuses, curl behavior is further influenced by the heat conductivity of the paper, moisture evaporation from the paper, and the other heating conditions. Thus, curl behavior is influenced by different types of paper as well as different apparatuses used with the same type of paper, resulting in the general unpredictability of the tendency of a paper to curl.
Generally, two types of curl may be generated in paper manufacture as a result of moistening or drying a paper sheet. Total (simplex) curl is when a paper curls in one direction, such as a sheet of paper rolled into a cylindrical tube. Diagonal curl results from a twist of paper, such that one, portion of the paper rotates axially in one direction, while another portion of the paper rotates axially in a different, generally opposite direction.
Current curl tests for prediction of curl tendency are problematic. One procedure involves running multiple sheets through a specified (Xerox 5388) photo copier, hanging a preset number of sheets by the long edge and matching the resulting bend to a pattern of curves on a reference template. This test, however, requires the use of specific, expensive equipment and is costly and time consuming. It also requires that papers be sheeted prior to testing, delaying results.
Hot plate curl is another method currently in use. It involves placing cut samples on a heated hotplate and manually measuring the corners of the sample as they curl away from the heat. This method is time intensive and has very poor (r^2 0.30) correlation to end use as well as poor reproducibility.
Other tests do not account for the ability of gravity to alter the curl of a paper, resulting in errant data. For example, other mechanisms, such as a computer controlled hot-roll fusing apparatus, are able to test for a variety of paper parameters by mimicking the environment of a typical consumer printer, yet the nips are horizontal and have too high nip pressure, resulting in gravitational and mechanical effects on the test samples.
Therefore, it is an object of this invention to provide a test for the tendency of a paper to curl from which information can be achieved quickly and possible corrective feedback is rapid, has a low level of variability, and accounts for the effect of gravity on a tendency to curl for a variety of paper samples.